Undetected BVOCs from Norway spruce drive total ozone reactivity measurements

Biogenic Volatile Organic Compounds (BVOCs) are continuously emitted from terrestrial vegetation into the atmosphere and react with various atmospheric oxidants, with ozone being an important one. The reaction between BVOCs and ozone can lead to low volatile organic compounds, other pollutants, and the formation of secondary organic aerosols. To understand the chemical and physical processes taking place in the atmosphere, a complete picture of the BVOCs emitted is necessary. However, the large pool of BVOCs present makes it difficult to detect every compound. The total ozone reactivity method can help understand the ozone reactive potential of all BVOCs emitted into the atmosphere and also help determine if current analytical techniques can measure the total BVOC budget. In this study, we measured the total ozone reactivity from the emissions of a Norway spruce tree in Hyytiälä in late summer using the Total Ozone Reactivity Monitor (TORM) built at the Finnish Meteorological Institute (FMI). We also conducted chemical characterisation and quantification of the BVOC emissions using a gas chromatograph coupled with a mass spectrometer (GC-MS). The measured total ozone reactivity reached up to 7.3 $\times$ 10$^{-9}$ \unit{m^{3}\,s^{-2}\,g^{-1}}, which corresponds to 64 \unit{\mu g\,g^{-1}\,h^{-1}} of \chem{\alpha}-pinene. Stress-related sesquiterpenes such as \chem{\beta}-farnesene and \chem{\alpha}-farnesene, and an unidentified sesquiterpene contributed the most to the observed emissions. However, the observed emissions made up only 35\% of the measured total ozone reactivity, with sesquiterpenes being the most important sink for the ozone. High total ozone reactivity was especially seen during high temperature periods, with up to 95\% of the reactivity remaining unexplained. Emissions of unidentified stress-related compounds could be the reason for the high fraction of missing reactivity